An amphoteric water-soluble polymer has been used as a water treatment agent in fields relevant to sludge dewatering treatment, retention rate improvers for raw materials of paper making, and the like. For example, with respect to sludge dewatering treatment, because of increase of the generation quantity of sludge and deterioration of sludge properties in these years, conventional cationic polymer dewatering agents are limited in the capabilities of treating sludge and are not necessarily capable of promoting treatment to a satisfactory extent in terms of the water content of a dewatered cake, the SS recovery ratio, the release property of the cake from a filter cloth, and the like, and thus it is urgent to improve the agents. In order to overcome the defects of such conventional cationic polymer dewatering agents, a variety of amphoteric polymer dewatering agents have been proposed, however these amphoteric polymer dewatering agents are not necessarily satisfactory. For example, (1) an amphoteric polymer dewatering agent having a tertiary amino group (Japanese Patent Application Laid-Open (JP-A) No. 62-205112), (2) an amphoteric polymer dewatering agent having a quaternary ammonium group (JP-A No. 53-149292), and (3) an amphoteric polymer dewatering agent having both tertiary and quaternary groups (JP-A No. 3-18900) are disclosed.
However, with respect to the foregoing amphoteric polymer dewatering agent (1), although it is excellent in flocculation property as compared with the conventional cationic polymer dewatering agent and forms large flocculated flock, its capability is considerably deteriorated for the sludge with high pH such as digestion sledges of wastewater and night soil since the tertiary amino group is not dissociated in a high pH range and it is significantly affected by fluctuation of the sludge properties such as the sludge concentration as well as pH and incapable of promoting stable treatment and it is also disadvantageous in terms of product stability in powder or solution state as compared with the conventional cationic polymer dewatering agent. With respect to the foregoing amphoteric polymer dewatering agent (2), although it is good in terms of the product stability as compared with the amphoteric polymer dewatering agent having a tertiary amino group and has high flocculation capability as compared with the conventional cationic polymer dewatering agent, it has many disadvantageous points to be improved: it is required to add them in a large quantity: the water contents of resulting cakes are high: separation property of cakes from filter cloths is inferior: and so forth. On the other hand, the foregoing amphoteric polymer dewatering agent (3) is improved in the disadvantageous properties which the agents (1) and (2) have, however it is not yet satisfactory in terms of the required addition quantity and the water content in cakes. Further, JP-A No. 7-256299 discloses an amphoteric polymer dewatering agent having a high methacrylate content and cationic group content, and especially JP-A No. 7-256300 discloses an amphoteric polymer dewatering agent having a high methacrylate content and anionic group content. Various polymers as sludge dewatering agents have been developed; nevertheless, there are still many problems to be solved for practical use.
Meanwhile, new movements have taken place in paper making industries. That is, recycling has become important and mixing ratio of waste paper has been increasing from a viewpoint of saving resources and preserving environments. Further, the ratio of the use of mechanical pulp, which can be produced in short process with a small amount of a chemical agent and suppressed generation of production wastewater as compared with chemical pulp, tends to be increased. The mechanical pulp, although its production cost is low, has a short pulp fiber and therefore, its retention rate on wires is inevitably decreased. Moreover, anionic substances extracted from raw material wood cannot be completely removed in relation to the technical aspect and cost performance and contaminate paper making raw materials. Accordingly, it is difficult to improve the retention rate by conventional retention process.
An object of the invention is to develop a polymer flocculant which can highly efficiently carry out dewatering treatment of sludges regardless of the seasonal fluctuation of the sludges; which can maintain a high retention rate even if the mixing ratio of waste paper and machine pulp is increased in the case of using the flocculent as a retention rate improver in paper making; and which has high flocculating capability and quick solubility as compared with a flocculent in powder state. For that, another object the invention is to develop a polymerization method for efficiently synthesizing an amphoteric polymer having cationic groups in a high concentration from an acrylate monomer containing a quaternary ammonium salt group and a methacrylate monomer containing a quaternary ammonium salt group with a high concentration. Further, another object of the invention is to provide various flocculation methods using the amphoteric polymer flocculent.